In an optical pickup device used to read or write data on a recording medium such as a compact disc (CD) or a digital versatile disc (DVD), as shown in FIG. 23A a light beam from a light source 1 is converted, by a collimator lens 2, into a substantially parallel beam of light which is focused by an objective lens 3 onto the recording medium 4, and an information signal is generated by receiving the light beam reflected from the recording medium 4. In such an optical pickup device, when reading or writing data on the recording medium, the light beam focused by the objective lens 3 must be made to accurately follow the track on the recording medium 4. However, a tilt can occur at the surface of the recording medium 4, due to warping or curving of the recording medium 4, imperfections in the driving mechanism for the recording medium 4, etc. If the optical axis of the light beam focused by the objective lens 3 is tilted relative to the track on the recording medium 4, coma aberration occurs in the substrate of the recording medium 4; that is, when seen at the entrance pupil position of the objective lens 3 (i.e., the position where a liquid crystal optical element 5 is to be inserted), coma aberration 20 such as shown in FIG. 23B occurs, causing a degradation of the information signal generated based on the light beam reflected from the recording medium 4.
In view of this, it has been proposed to correct the coma aberration associated with the tilting of the recording medium 4, by placing a liquid crystal optical element 5 in the path between the collimator lens 2 and the objective lens 3 as shown in FIG. 24 (refer, for example, to Patent Publication 1). That is, utilizing the property that the orientation of liquid crystal molecules changes according to the potential difference caused in the liquid crystal, the liquid crystal optical element 5 works to change the phase of the light beam passing through the liquid crystal and thereby cancel out the coma aberration.
FIG. 25A shows a transparent electrode pattern 30 for generating a phase distribution in the liquid crystal according to the voltage applied to the coma aberration correcting liquid crystal optical element 5. In FIG. 25A, two regions 32 and 33 for advancing the phase and two regions 34 and 35 for delaying the phase are formed in a region having approximately the same size as the effective diameter 10 of the light beam incident on the liquid crystal optical element 5. In the figure, reference numeral 31 indicates a region to which a reference potential is applied.
When a positive (+) voltage is applied to the regions 32 and 33, a potential difference occurs with respect to a transparent electrode on the opposite side (not shown), and the orientation of the liquid crystal molecules therebetween changes according to the potential difference. As a result, in the case of a conventional p-type liquid crystal, the light beam passing therethrough is acted upon by a force that advances its phase. On the other hand, when a negative (−) voltage is applied to the regions 34 and 35, a potential difference occurs with respect to the transparent electrode on the opposite side (not shown), and the orientation of the liquid crystal molecules therebetween changes according to the potential difference. As a result, in the case of a conventional p-type liquid crystal, the light beam passing therethrough is acted upon by a force that delays its phase. The reference potential (here, 0 V as an example) is applied to the region 31. These voltages are applied to the transparent electrode pattern 30 via a lead 6 (see FIG. 24).
In FIG. 25B, the voltages 21 applied to the respective regions are plotted on the X axis. The coma aberration 20 is corrected by applying suitable voltages to the transparent electrode pattern 30. FIG. 25C shows the coma aberration 22 after the correction. As shown in FIG. 25C, by using the liquid crystal optical element 5, corrections are made so as to suppress the coma aberration occurring in the substrate of the recording medium 4.
However, in addition to the problem associated with the tilting of the recording medium 4, there also occurs the problem of the optical axis of the objective lens 3 becoming displaced from the track on the recording medium 4 (optical axis displacement). To address this, as shown in FIG. 26, the objective lens 3 is attached to a tracking actuator 7 by which the optical axis of the objective lens 3 is made to follow the track on the recording medium 4. The actuator 7 has a power supply lead 8. With the actuator 7 moving the objective lens 3 in directions indicated by arrow A in the figure, the light beam focused by the objective lens 3 is corrected so as to accurately follow the track on the recording medium 4 (in FIG. 26, the light beam 11 is corrected as shown by a light beam 12).
However, when the objective lens 3 is moved by the actuator 7, the positional relationship between the liquid crystal optical element 5 and the objective lens 3 changes. On the other hand, the phase-modulating transparent electrode pattern 30 (FIG. 25A) formed in the liquid crystal optical element 5 is designed so as to match the effective diameter 10 of the optical pickup device. That is, the liquid crystal optical element 5 is designed so that the coma aberration occurring in the substrate of the recording medium 4 can be ideally corrected only when the objective lens 3 and the liquid crystal optical element 5 are precisely aligned along the optical axis. Accordingly, with the positional relationship between the liquid crystal optical element 5 and the objective lens 3 deviated from the ideal condition, if a tilt occurs at the surface of the recording medium 4, the coma aberration cannot be sufficiently corrected by the liquid crystal optical element 5.
In view of this, it has been proposed to mount the phase-modulating liquid crystal optical element 5 integrally with the objective lens 3 and move them as a unit by the same actuator 7 as shown in FIG. 26 (refer, for example, to Patent Publication 2).
However, mounting the phase-modulating liquid crystal optical element 5 integrally with the objective lens 3 and moving them as a unit by the same actuator 7 involves the following problems.
First, when the phase-modulating liquid crystal optical element 5 is mounted integrally, the weight acting on the actuator 7 increases. The actuator 7 is required to move the objective lens 3 at an extremely fast speed or within several milliseconds, but the additional weight of the liquid crystal optical element 5 decreases the ability of the actuator 7 to move the objective lens 3 so as to follow the track on the recording medium 4. Secondly, the liquid crystal optical element 5 must be provided with the lead 6 for driving the liquid crystal optical element 5 but, because of the provision of the lead 6, the spring rate changes, which increases the complexity of the control for operating the objective lens 3 and the liquid crystal optical element 5 constructed as a single unit. In particular, there arises a concern that the lead 6 may be entangled, and interfere, with the tracking action of the objective lens 3.
Further, in an optical pickup device used to read or write data on a recording medium such as a DVD or a next generation high density DVD, as shown in FIG. 27A a light beam from a light source 1 is converted by a collimator lens 2 into a substantially parallel beam of light which is focused by an objective lens 3 onto the recording medium 4, and an information signal is generated by receiving the light beam reflected from the recording medium 4. When reading or writing data on the recording medium 4 using such an optical pickup device, the light beam must be accurately focused, by the objective lens 3, onto the track on the recording medium 4.
However, due to such factors as unevenness in the thickness of the light transmissive protective layer (indicated by B in FIG. 27A) formed on the track surface of the recording medium 4, the distance from the objective lens 3 to the track surface may not be constant at all times, or the light spot may not be able to focus identically at all times. Furthermore, in cases where more than one track surface is formed in the recording medium 4 to increase the storage capacity of the recording medium 4, there also arises a need to adjust the positional relationship between the objective lens 3 and each track surface.
In this way, if a variation occurs in the distance between the objective lens 3 and the track surface, spherical aberration occurs in the substrate of the recording medium 4, resulting in a degradation of the information signal generated based on the light beam reflected from the recording medium 4. FIG. 27B shows one example of the spherical aberration 23 when seen at the entrance pupil position of the objective lens 3. On the other hand, in cases where more than one track surface is formed in the recording medium, spherical aberration occurs when reading or writing data on the second track surface which is located at a position different from that of the first track surface located at the focal point of the objective lens 3, and this also causes a degradation of the information signal generated based on the light beam reflected from the recording medium 4.
In view of this, it has been proposed to correct the spherical aberration, occurring in the substrate of the recording medium, by placing a liquid crystal optical element 5 in the path between the collimator lens 2 and the objective lens 3 as shown in FIG. 28 (refer, for example, to Patent Publication 3). That is, utilizing the property that the orientation of liquid crystal molecules changes according to the potential difference applied to the liquid crystal, the liquid crystal optical element 5 works to change the phase of the light beam passing through the liquid crystal and thereby cancel out the spherical aberration.
FIG. 29A shows one example of a transparent electrode pattern 40 for generating a phase distribution in the liquid crystal according to the voltage applied to the spherical aberration correcting liquid crystal optical element 5. In FIG. 29A, nine concentric electrode patterns 41 to 49 are formed within the range of the effective diameter 10. Voltages 24 such as shown in FIG. 29B are applied to the respective regions. When the voltages shown in FIG. 29B are applied to the electrode pattern 40 shown in FIG. 29A, a potential difference occurs with respect to the transparent electrode on the opposite side, and the orientation of the liquid crystal molecules therebetween changes according to the potential difference. As a result, the light beam passing therethrough is acted upon by a force that advances its phase according to the potential difference. Thus, the spherical aberration 23 occurring in the substrate of the recording medium 4 is corrected as shown by a spherical aberration 25 in FIG. 29C. The voltages are applied to the transparent electrode pattern 40 via a lead 6 (see FIG. 28).
However, in addition to the problem of the spherical aberration occurring in the substrate of the recording medium 4 described above, there also occurs the problem of the optical axis of the objective lens 3 becoming displaced from the track on the recording medium 4 (axis displacement). To address this, as shown in FIG. 30, the objective lens 3 is attached to a tracking actuator 7 by which the optical axis of the objective lens 3 is made to follow the track on the recording medium 4. The actuator 7 has a power supply lead 8. With the actuator 7 moving the objective lens 3 in directions indicated by arrow A in the figure, the light beam focused by the objective lens 3 is made to accurately follow the track on the recording medium 4.
However, when the objective lens 3 is moved by the actuator 7, the positional relationship between the liquid crystal optical element 5 and the objective lens 3 changes. On the other hand, the transparent electrode pattern 40 (see FIG. 29A) formed in the liquid crystal optical element 5 is designed so as to match the effective diameter 10 of the optical pickup device. That is, the liquid crystal optical element 5 is designed so that the spherical aberration occurring in the substrate of the recording medium 4 can be ideally corrected only when the objective lens 3 and the liquid crystal optical element 5 are precisely aligned along the optical axis. Accordingly, when the positional relationship between the liquid crystal optical element 5 and the objective lens 3 is deviated from the ideal condition, the spherical aberration cannot be sufficiently corrected by the liquid crystal optical element 5.
Here, if the phase-modulating liquid crystal optical element 5 is mounted integrally with the objective lens 3 so that they can be moved as a unit by the same actuator 7 as shown in FIG. 30, problems similar to those described with reference to FIG. 26 occur; first, when the phase-modulating liquid crystal optical element 5 is mounted integrally, the weight acting on the actuator 7 increases. The actuator 7 is required to move the objective lens 3 at an extremely fast speed or within several milliseconds, but the additional weight of the liquid crystal optical element 5 decreases the ability of the actuator 7 to move the objective lens 3 so as to follow the track on the recording medium 4. Secondly, the liquid crystal optical element 5 needs to be provided with the lead 6 for driving the liquid crystal optical element 5 but, because of the provision of the lead 6, the spring rate changes, which increases the complexity of the control for operating the objective lens 3 and the liquid crystal optical element 5 constructed as a single unit. In particular, there arises a concern that the lead 6 may be entangled, and interfere, with the tracking action of the objective lens 3.
It is also known to provide an optical apparatus, such as shown in FIG. 31, that can play back a high density optical disc 707 having a 0.6-mm thick transparent substrate, such as DVD, as well as an optical disc 708 having a 1.2-mm thick transparent substrate, such as compact disc (CD), by using an optical disc device having a single objective lens 113 (refer, for example, to Patent Publication 4).
In FIG. 31, light source 1 is a light source for a high density optical disc and it emits a light beam with a wavelength of 650 nm. The light beam emitted from the light source 1 is converted by a collimator lens 2 into a substantially parallel beam of light, which is limited by an aperture 57 to a light beam having an effective diameter 110 which is about 5 mm in diameter; the light beam is then passed through a half mirror 56 and enters the objective lens 113. The objective lens 113 is an objective lens with a numerical aperture (NA) of 0.65, for a high density optical disc, and focuses the incident light beam onto the high density optical disc 707 having a 0.6-mm transparent substrate.
On the other hand, a light source 101 is a light source for CD, and emits a light beam with a wavelength of 780 nm. The light beam emitted from the light source 101 is converted by a collimator lens 102 into a substantially parallel beam of light, which is limited by an aperture 58 to a light beam having an effective diameter 120 which is about 4 mm in diameter; the light beam is then redirected by the half mirror 56 and enters the objective lens 113. The objective lens 113 focuses the incident light beam onto the optical disc 708 having a 1.2-mm transparent substrate.
By switching between the two light sources according to the kind of the optical disc to be played back, two kinds of optical discs can be played back using the single objective lens 113.
However, a tilt can occur at the surface of the optical disc 707 (disc tilt), due to warping or curving of the optical disc 707, imperfections in the driving mechanism for the optical disc 707, etc. Because of such disc tilt, wavefront aberration (primarily, coma aberration) occurs in the substrate of the optical disc 707 when reading or writing data on the optical disc 707.
The coma aberration 20 occurring in the substrate of the optical disc 707, when represented by the pupil coordinate of the objective lens 113, is as shown in FIG. 23B. The coma aberration causes a degradation of the light intensity signal created based on the light beam reflected from the optical disc 707. For the optical disc 708 also, disc tilt can occur but, usually, the necessity of correction is low because of low recording density, etc.
Further, the objective lens 113 in such an optical apparatus is configured to focus a light spot on the track surface of the high density optical disc 707 having a 0.6-mm transparent substrate; therefore, if the light spot is to be focused on the track surface of the optical disc 708 such as CD having a 1.2-mm transparent substrate, the light spot cannot be accurately focused on the track even if the effective diameter of the incident light beam is reduced. This therefore results in the generation of spherical aberration in the substrate when playing back the optical disc 708 such as CD having a 1.2-mm transparent substrate.
The spherical aberration 23 occurring in the substrate of the optical disc 708 such as CD having a 1.2-mm transparent substrate, when represented by the pupil coordinate of the objective lens 113, is as shown in FIG. 27B. The spherical aberration causes a degradation of the light intensity signal created based on the light beam reflected from the optical disc 708.
(Patent Publication 1)
Japanese Unexamined Patent Publication No. 2001-143303 (page 3, FIG. 1)
(Patent Publication 2)
Japanese Unexamined Patent Publication No. 2000-215505 (page 2, FIG. 1)
(Patent Publication 3)
Japanese Unexamined Patent Publication No. H10-269611 (pages 3 to 5, FIGS. 1 to 3, FIG. 5)
(Patent Publication 4)
Japanese Unexamined Patent Publication No. 2001-101700 (pages 5 to 6, FIG. 6)